The application generally relates to an electrical relay. The application relates more specifically to a relay with integral phase controlled switching for reduced surge current adaptable to changing conditions and different relays to optimize and/or reduce manufacturing cost.
AC powered capacitance and inductive loads may be difficult to control with a conventional relay. If the relay contacts are closed at the peak voltage of the power line a large inrush current may be created as the capacitive load charges to the peak voltage. The only factors for limiting the inrush current are provided by parasitic inductance and resistance in the load.
Integral phase controlled switching of the relay contacts, as disclosed below, ensures that the relay contacts only close at the zero-crossover point of the input voltage, i.e., when the voltage across the relay contacts is substantially zero. The sinusoidal waveform of the voltage limits the rate of rise in the current on the capacitive load.
In addition, some inductive loads such as transformers are designed so the current drives the flux density of the core close to saturation. The steady state flux is driven positive and negative alternately. During start up the flux in the core is zero and if the contacts are closed coincident with the zero crossing, flux increases for a full half cycle of the line voltage. This drives flux much higher than the steady state and exceeds the core saturation. Once the transformer core is saturated the inductance rapidly decreases and the current is only limited, e.g., by resistance of connecting wires and the transformer primary winding resistance. For transformers in the 1000 VA range the current can exceed 100 amperes. If the relay contacts close at peak line voltage, the core flux has only ¼ cycle to build up magnetic flux. The voltage polarity then reverses and the flux density of the full sine wave half-cycle does not reach peak value. The flux density fails to reach peak value because the transformer core has an opposing value built up from the previous ¼ cycle, i.e., the input voltage is not initially zero. When this occurs core saturation and high currents are prevented.
Traditional electrical relays have many difficulties in obtaining reliable operation over an extended operational life. If the connected load is not compatible with the relay, the relay contacts may be degraded by due to burning and sticking contacts, caused by arcing when the relay contacts are switched with current flowing between them.
Many applications require that the right range of voltage and current drive be available, placing additional requirements on the system design.
For a large system with many relays involved there are many wires and drivers required in the design of the system.
Excessive overload currents can lead to overheating and failure of the relays.
Other problems present in existing electrical relays include high current drive. Also, overheating of the relay can occur from failure of the relay contacts due to burning and sticking.
Therefore, to achieve greater relay life, it is desirable to reduce or eliminate arcing and inrush current upon the opening or closing of the relay contacts. One method of reducing or eliminating arcing upon the opening or closing of the relay contacts is to control the operation of the relay to open or close at a zero current point in the AC load circuit to eliminate arcing at the relay contacts.
Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments that fall within the scope of the claims, regardless of whether they accomplish one or more of the aforementioned needs.